Fusidic acid [CAS6990-06-3] [Nature, Vol. 193, No. 4819, p. 987, 1962] which can be isolated from the fermentation broth of Fusidium coccineum is the most antibiotically active compound of the fusidanes and is the only fusidane used clinically in treatment of infectious diseases. Fusidic acid (Fucidin®) is used clinically for the treatment of severe staphylococcal Infections, particularly in bone and joint Infections, in both the acute and the intractable form of the disease (The Use of Antibiotics, 5th Ed., A. Kucers and N.McK. Bennett (Eds.), Butterworth 1997, pp. 580-587, and references cited therein).

Although fusidic acid is most commonly used against staphylococci, it is also used against several other gram-positive species. The clinical value of fusidic acid is also due to its efficient distribution in various tissues, low degree of toxicity and allergic reactions and the absence cross-resistance with other clinically used antibiotics. Fusidic acid is widely used in local therapy for a number of skin and eye infections caused by staphylococci. It is generally given in combination with common antibiotics such as penicillins, erythromycins or clindamycin. It has also been used as an alternative to vancomycin for the control of Clostridlum diffcile. Compared to staphylococci, several other gram-positive cocci are often less susceptible to fusidic acid. As an example, streptococcal species are generally up to 100-fold less sensitive to fusidic acid than staphylococci [Kuchers et al; supra]. Other sensitive bacteria include gram-positive anaerobic cocci, such as Peptococcus and Peptostreptococcus spp., aerobic or anaerobic gram-positive bacteria, such as Corynebacterium diphtherdae, Clostridium tetani, Clostridium difficile and Clostridlum perfringens. Gram-negative bacteria are resistant except for Neisseria spp. and Legionella pneumophila. The drug is highly potent against both intracellular and extracellular M. leprae. 
The solid form, such as the crystal form, of a drug substance or active pharmaceutical ingredient used in a pharmaceutical formulation or medicament is important based on solubility, dissolution rate, hygroscopicity, bioavailability, and stability differences between the different solid forms. Thus the existence of various solid forms, such as polymorphism or pseudo polymorphism can affect the properties of the quality of the drug product.
Hence, a specific crystal form, including solvates and hydrates, might be preferable over another one. Furthermore certain forms may be preferable depending on the specific formulation and/or application. For example, the properties of a drug, such as the dissolution rate of the active ingredient, may be tuned by the proper choice of a certain crystal form, or mixtures thereof.
In a specific commercial pharmaceutical formulation, fusidic acid is presently marketed [see Monographs in the European Pharmacopeia 5.0] as a hemihydrate, which is the only hemihydrate form which has been described.
Patent GB 930,786 discloses salts of fusidic acid with organic and inorganic bases, solvates of fusidic acid, namely a benzene solvate and a methanol solvate. This patent further discloses an unspecified fusidic acid form with IR absorption bands (KBr) at 1265, 1385, 1695, 1730 and 3450 cm−1 and having a specific rotation [α]D22 of minus 9 degrees (1% solution in CHCl3) obtainable by crystallisation of the methanol solvate of fusidic acid from ether. However, this form is distinct from the form of the present invention evident from the depicted IR spectrum in GB 930,786 which indicates that this form actually corresponds to the presently marketed hemihydrate form.
Solvates and salts of fusidic acid have also been disclosed in British patent GB 999,794. Patent ES 2208110 discloses two solvent free crystalline forms of fusidic acid called Form I and Form II, and a crystalline hemihydrate called Form III which is identical to the presently marketed hemihydrate, respectively. The crystalline forms were identified and characterised by IR spectroscopy, differential scanning calorimetry, X-ray diffraction and melting points.
Patent WO 96/03128 discloses tablets containing a sodium salt form of fusidic acid and WO 86/03966 describes an ophthalmic gel composition comprising an undefined form of suspended fusidic acid.